A stock interface can be slipped onto and attached to a standard CAR buffer tube. The stock interface provides structures for attaching a butt stock and stock interface attachments such as an ergonomic attachment, a container attachment, and a power attachment. Small arms ergonomics can be improved by providing a better cheek weld via an ergonomic attachment. Container attachments provide storage. Power attachments provide electrical power for items such as electronic sighting systems, lasers, lights, or any of the other electronic devices attached to a small arm.